Magnetic properties of solid solutions between BiCrO3 and BiGaO3 with perovskite structures.

Magnetic properties of BiCr1-x Ga x O3 perovskite-type solid solutions are reported, and a magnetic phase diagram is established. As-synthesized BiCrO3 and BiCr0.9Ga0.1O3 crystallize in a monoclinic (m) C2/c structure. The Néel temperature (TN) decreases from 111 K in BiCrO3 to 98 K in BiCr0.9Ga0.1O3, and spin-reorientation transition temperature increases from 72 K in BiCrO3 to 83 K in BiCr0.9Ga0.1O3. o-BiCr0.9Ga0.1O3 with a PbZrO3-type orthorhombic structure is obtained by heating m-BiCr0.9Ga0.1O3 up to 573 K in air; it shows similar magnetic properties with those of m-BiCr0.9Ga0.1O3. TN of BiCr0.8Ga0.2O3 is 81 K, and TN of BiCr0.7Ga0.3O3 is 63 K. Samples with x = 0.4, 0.5, 0.6 and 0.7 crystallize in a polar R3c structure. Long-range antiferromagnetic order with weak ferromagnetism is observed below TN = 56 K in BiCr0.6Ga0.4O3, TN = 36 K in BiCr0.5Ga0.5O3 and TN = 18 K in BiCr0.4Ga0.6O3. BiCr0.3Ga0.7O3 shows a paramagnetic behaviour because the Cr concentration is below the percolation threshold of 31%.

Introduction

Bi-containing perovskites have received a lot of attention as multiferroic materials and lead-free ferroelectrics [1-4]. The stereochemically active 6s2 lone pair of a Bi3+ ion plays an important role in producing polar distortions in Bi-containing perovskites. They form a basis for materials with super-tetragonality and a huge spontaneous polarization (PS) observed, for example, in BiCoO3 [4], Bi2ZnTiO6 [5], Bi2ZnVO6 [6], BiCo0.3Fe0.7O3 [7], and highly strained BiFeO3 thin films [3, 8]. Bulk BiFeO3 [3] and BiAlO3 [4] also crystallize in a polar structure with space group R3c and large PS.

On the other hand, BiCrO3 [9, 10] crystallizes in a centrosymmetric structure with space group C2/c [4, 11–13]. There is a structural phase transition to a GdFeO3-type Pnma structure from 420 K in BiCrO3 [9-12]. It is interesting that during the Pnma-to-C2/c phase transition, twin domains with the size of less than 10 nm are formed [12, 14]. First-principle calculations could not explain the experimental C2/c structure and predicted the GdFeO3-type Pnma structure as the ground-state structure [15, 16]. The Néel temperature (TN) of BiCrO3 is 109–111 K [9–13, 17], and there is a spin-reorientation transition below 75–80 K in BiCrO3 [12, 13, 17].

Among BiMO3 (M = 3d transition metals) compounds, BiCrO3 is one of the least studied compounds, especially its solid solutions with isovalent substitutions. From the BiCrO3-rich side, just a few selected compositions have been investigated in Bi1−YCrO3 (x = 0.01, 0.05, 0.2 and 0.5 [13] and 0.1 [18]), BiCr1−FeO3 (x = 0.5 [19]) and BiCr1−MnO3 (x = 0.5 [20]) bulk systems. We have recently identified BiGaO3-based perovskites, BiM1−GaO3 (M = Cr, Mn and Fe), as a large family of polar materials, which includes phases with (pseudo) super-tetragonality and R3c symmetry [21]. In particular, the R3c polar phase was found at 0.4 ≤ x ≤ 0.7 in BiCr1−GaO3 solid solutions even though the end members, BiCrO3 [4, 11] and BiGaO3 [4], crystallize in centrosymmetric crystal structures. In this work, we report on detailed magnetic properties of the BiCr1−GaO3 solid solutions.

Experimental section

BiCr1−GaO3 with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 were prepared from stoichiometric mixtures of Bi2O3 (99.9999%), Cr2O3 (99.99%) and Ga2O3 (99.99%). The mixtures were reground under acetone several times, placed in Pt capsules, dried at 573 K for several days and finally treated in a belt-type high-pressure apparatus at 6 GPa and 1700 K for 2 h (heating rate to the desired temperature was 10–15 min). After heat treatment, the samples were quenched to room temperature (RT), and the pressure was slowly released. Samples with other x values were mixtures of two perovskite phases (R3c and Cm phases for x = 0.8) and perovskite (Cm) and pyroxene phases for x = 0.9 [21]; therefore, their magnetic properties were not studied.

X-ray powder diffraction (XRPD) data were collected at RT on a RIGAKU Ultima III diffractometer using CuK radiation (2θ range of 10°–120°, a step width of 0.02°, and a counting time of 2–14 s/step). The samples contained small amounts of Cr2O3 and Bi2O2CO3 impurities. The formation of Bi2O2CO3 impurity was observed in many other works [13], and it is usually attributed to the diffusion of carbon from carbon heaters through cracks in capsules. Cr2O3 impurity could remain because a small amount of Bi2O3 was removed as Bi2O2CO3 from stoichiometric mixtures.

Magnetic susceptibilities (χ = M/H) were measured using pellets on SQUID magnetometers (Quantum Design, MPMS XL and 1 T) between 2–5 and 300–400 K in different applied fields. Samples were rapidly inserted into magnetometers kept at 10 K and having a zero magnetic field; then, temperature was set to 2 or 5 K; at 2 or 5 K, measurement magnetic fields were applied; and finally measurements were performed on heating up to 300, 350, or 400 K. This procedure gave zero-field-cooled (ZFC) curves. After ZFC measurements, samples were measured on cooling resulting in field-cooled (FC) curves. Isothermal magnetization measurements were performed between −50 and 50 kOe or between −10 and 10 kOe at different temperatures. Frequency dependent ac susceptibility measurements at a zero static magnetic field were performed with a Quantum Design MPMS 1 T instrument from 150 to 2 K at frequencies (f) of 2, 110, and 300 Hz and an applied oscillating magnetic field (Hac) of 5 Oe. Specific heat, Cp, at a zero magnetic field was recorded between 2 and 300 K on cooling by a pulse relaxation method using a commercial calorimeter (Quantum Design PPMS).

Differential scanning calorimetry (DSC) curves were recorded on a Mettler Toledo DSC1 STARe system at a heating/cooling rate of 10 K min−1 from 290 to 573 K in open aluminium capsules; three cycles were performed to check the reproducibility.

Results and discussion

As-synthesized BiCrO3 and m-BiCr0.9Ga0.1O3 crystallize in the monoclinic (m) C2/c structure. The lattice parameters refined by the Rietveld method are a = 9.4786(4) Å, b = 5.4852(2) Å, c = 9.5824(4) Å and β = 108.587(3)° for BiCrO3 and a = 9.4762(4) Å, b = 5.4899(2) Å, c = 9.5791(4) Å and β = 108.571(3)° for m-BiCr0.9Ga0.1O3. However, both BiCrO3 and BiCr0.9Ga0.1O3 show noticeable anisotropic broadening (AB) of some reflections, the appearance of continuous diffuse scattering (DS) between some reflections, and shifts of some reflections (RS) from their ideal/expected positions (figure 1). Those features are most probably originated from local disorder, the presence of nanodomains [12, 14] and high concentration of domain boundaries and defects; they make the precise Rietveld analysis impossible and result in large variations in refined lattice parameters depending on models used. Similar features are observed in as-synthesized m-BiCr0.8Ga0.2O3 and m-BiCr0.7Ga0.3O3 (figures S1–S3 of the electronic supporting information (ESI)). If those features are modelled as a second perovskite phase (a PbZrO3-related phase with space group Pnma [21]) in the Rietveld analysis, the weight fraction of the second phase is estimated to be about 20% in BiCrO3, 20% in m-BiCr0.9Ga0.1O3, 30% in m-BiCr0.8Ga0.2O3 and 50% in m-BiCr0.7Ga0.3O3 [21]. Electron microscopy and electron diffraction studies showed that the AB, DS and RS features are intrinsic for BiCrO3 and do not originate from the presence of other perovskite phases [12, 14]. They can also be considered as intrinsic in m-BiCr0.9Ga0.1O3 based on the above numbers. However, for unambiguous interpretation of those features in m-BiCr0.8Ga0.2O3 and m-BiCr0.7Ga0.3O3 detailed high-resolution electron microscopy studies are needed.

Figure 1.

Portions of experimental (crosses) and calculated (lines) x-ray powder diffraction patterns of as-synthesized BiCrO3, m-BiCr0.9Ga0.1O3 and m-BiCr0.8Ga0.2O3 measured with the CuKα radiation at room temperature. Possible Bragg positions of the C2/c structure are indicated by tick marks. ‘AB’ means reflections with anisotropic broadening, ‘DS’ means diffuse scattering between some reflections, and ‘RS’ means reflection shifts from their ideal expected positions.

m-BiCr0.9Ga0.1O3 exhibits a structural phase transition and shows a peak on the DSC curve at about 500 K on heating, and at 480 K on cooling. After heating as-synthesized m-BiCr0.9Ga0.1O3 up to 573 K, its XRD pattern changes. All fundamental perovskite reflections remain the same; however, weak superstructure reflections change. Reflections can be indexed in the PbZrO3-related structure [21] with space group Pnma and lattice parameters of a = 5.4892(4) Å, b = 15.4761(9) Å and c = 11.1269(7) Å (figure 3); this sample will be called o-BiCr0.9Ga0.1O3. Irreversible transformations of high-pressure phases were also found, for example, in BiFe0.5Sc0.5O3 [22]. The origin of the irreversible behaviour is the existence of competing phases.

Figure 3.

Portions of experimental x-ray powder diffraction patterns of (a) m-BiCr0.9Ga0.1O3 and (b) o-BiCr0.9Ga0.1O3 measured with the CuKα radiation at room temperature in the logarithmic scale to emphasize the difference in weak reflections. Possible Bragg positions are indicated by tick marks for the main perovskite phases and Bi2O2CO3 and Cr2O3 impurities.

Figure 2.

Differential scanning calorimetry curves of BiCr0.9Ga0.1O3 measured at a heating/cooling rate of 10 K min−1 between 290 and 573 K. Results of three heating–cooling cycles are shown.

BiCrO3 has the Néel temperature (TN) of 111 K (determined/defined here by peak positions on the FC dχ/dT versus T curves at 100 Oe). Below TN, the magnetic moments of Cr3+ ions in BiCrO3 are aligned along the a axis [14] in the G-type antiferromagnetic (AFM) structure, where all Cr–O–Cr interactions are AFM. BiCrO3 has a spin-reorientation transition (TSR) at 72 K (also defined by peak positions on the FC dχ/dT versus T curves at 100 Oe), where Cr3+ spins start to rotate away from the a axis in the (a, c) plane [13], but keeping the G-type AFM arrangement. Characteristic anomalies at TN and TSR can be clearly seen on the χ versus T, dχ/dT versus T and χ′ versus T curves of BiCrO3 and m-BiCr0.9Ga0.1O3 (figures 4(a), (b) and 5). We note that small anomalies are observed at 165 K in BiCrO3 on the 100 Oe FC χ−1 versus T curve (figure 4(c)); they were suggested to originate from a very small amount of the GdFeO3-type Pnma modification of BiCrO3 [4, 18]. We note that those anomalies at 165 K with different magnitudes were observed in all checked BiCrO3 samples (about a dozen of different samples (figure S17 of ESI), even synthesized by different groups); and those anomalies cannot be eliminated by further annealing and very slow cooling [18] suggesting that they are ‘intrinsic’ for bulk BiCrO3 samples. As-synthesized m-BiCr0.9Ga0.1O3 shows very similar magnetic behaviour with that of BiCrO3, but with TN = 98 K and TSR = 83 K. No additional magnetic anomalies above TN are found in m-BiCr0.9Ga0.1O3 in comparison with BiCrO3; it can be related with the higher temperature of the C2/c-to-Pnma transition in BiCr0.9Ga0.1O3 that results in a complete transformation. Magnetic properties of m-BiCr0.9Ga0.1O3 and o-BiCr0.9Ga0.1O3 are very similar with each other (figures 4 and S13 of ESI). Inverse magnetic susceptibilities of BiCrO3, m-BiCr0.9Ga0.1O3 and o-BiCr0.9Ga0.1O3 are given on figure 4(c). They show a noticeable deviation from the Curie–Weiss behaviour far above TN; this is why the Curie–Weiss fits are performed above 250 K for those samples (table 1). This fact can also explain why the effective magnetic moments are slightly larger than expected ones.

Figure 4.

Magnetic properties of BiCr1−GaO3 (x = 0, m—0.1 and o—0.1). (a) ZFC (filled symbols) and FC (empty symbols) dc magnetic susceptibility curves measured at 100 Oe. (b) FC dχ/dT versus T curves at 100 Oe; peaks define phase transition temperatures. (c) ZFC and FC χ−1 versus T curves measured at 100 Oe.

Figure 5.

Real parts of the ac susceptibility curves of BiCr1−GaO3 (x = 0 [17] and m—0.1). TN is the Néel temperature, and TSR is temperature of a spin-reorientation transition. The data for m-BiCr0.9Ga0.1O3 are shifted by +0.01 cm3/mol-Cr for the clarity.

Table 1.

Temperatures of intrinsic magnetic transitions and results of the Curie–Weiss fits for as-synthesized BiCr1−GaO3.

Sample	TN (K)	TSR (K)	μeff per Cr3+	θ
BiCrO3	111	72	4.161(14)μB	−351(5) K
m-BiCr0.9Ga0.1O3	98	83	4.069(10)μB	−282(3) K
m-BiCr0.8Ga0.2O3	81		3.965(4)μB	−239(1) K
m-BiCr0.7Ga0.3O3	63		4.116(10)μB	−239(3) K
BiCr0.6Ga0.4O3	56		3.839(3)μB	−148.6(6) K
BiCr0.5Ga0.5O3	36		3.884(1)μB	−119.8(2) K
BiCr0.4Ga0.6O3	18		3.889(2)μB	−96.3(3) K
BiCr0.3Ga0.7O3	No		3.828(3)μB	−73.3(5) K

μeff is an effective magnetic moment, θ is the Curie–Weiss temperature. The calculated μeff for Cr3+ (S = 3/2) is 3.87μB.

For the Curie–Weiss fits, the FC curves at 10 kOe are used, and the data are corrected for diamagnetic contributions from sample holders and core diamagnetism. The Curie–Weiss fits are performed between 250 and 400 K for x = 0 and 0.1, 250 and 340 K for x = 0.2 and 0.3 and 150 and 400 K for x = 0.4–0.7 (figure S9 of the ESI).

Isothermal magnetization curves of BiCr0.9Ga0.1O3 are given on figure 6. They show that a very weak ferromagnetic moment is developed below TN in m-BiCr0.9Ga0.1O3; below TSR, the hysteresis loop becomes more defined. The magnetization reaches 0.074μB/Cr at 5 K and 50 kOe; at 5 K, the remnant magnetization is 0.0177μB/Cr, and the coercitive field is about 1.8 kOe in m-BiCr0.9Ga0.1O3. Very similar M versus H curves were obtained in BiCrO3 [17]. Spin canting angles in BiCrO3 could not be determined from neutron diffraction because of their small values [12, 13].

Figure 6.

(a) Isothermal magnetization curves of m-BiCr0.9Ga0.1O3 and o-BiCr0.9Ga0.1O3 at 5 K. (b) Isothermal magnetization curves of m-BiCr0.9Ga0.1O3 at 50 and 90 K. Insert shows details near the origin.

Detailed magnetic properties (χ versus T, χ−1 versus T, dχ/dT versus T, χ′ versus T, χ″ versus T, M versus H and Cp/T versus T) of as-synthesized m-BiCr0.8Ga0.2O3 and m-BiCr0.7Ga0.3O3 are given in figures S8–S11 of the ESI. TN is found to be 81 K in BiCr0.8Ga0.2O3 and 63 K in BiCr0.7Ga0.3O3 from dχ/dT versus T and Cp/T versus T curves. TN remains the same (figure S13 of ESI) for the as-synthesized samples and samples after DSC experiments (up to 623 K for BiCr0.8Ga0.2O3 and 773 K for BiCr0.7Ga0.3O3 [21]). Magnetic entropy is almost independent of x for 0.0 ≤ x ≤ 0.3 and varies between 5.2 and 5.5 J K−1mol−1 (figure S14 of the ESI).

The χ versus T curves of BiCr1−GaO3 with x = 0.4, 0.5, 0.6 and 0.7 having the R3c polar symmetry are shown on figure 7, and the parameters of the Curie–Weiss fits are summarized in table 1 (the fits are given in figure S9 of the ESI). Effective magnetic moments per a Cr3+ ion are close to the expected value for those samples. The temperature of magnetic transitions in BiCr1−GaO3 monotonically decreases with increasing x indicating that the magnetic transitions are intrinsic. The ZFC and FC curves of BiCr0.6Ga0.4O3 are typical for canted antiferromagnets; in particular, the FC curves demonstrate the saturation behaviour. In addition, BiCr0.6Ga0.4O3 shows a weak specific heat anomaly at 55 K (figure 8). These features indicate that there is a long-range magnetic ordering in BiCr0.6Ga0.4O3. The χ′ versus T and χ″ versus T curves of BiCr0.6Ga0.4O3 (figure 9(a)) show sharp and frequency-independent peaks at TN with additional very broad anomalies near 22 K (on the χ″ versus T curves).

Figure 7.

ZFC (filled symbols) and FC (empty symbols) dc magnetic susceptibility curves of BiCr1−GaO3 (R3c) (x = 0.4, 0.5, 0.6 and 0.7) measured at 100 Oe. The insert gives the ZFC and FC χ versus T curves for x = 0.4 and 0.5 measured at 1 kOe.

Figure 8.

Specific heat data of BiCr1−GaO3 at 0 Oe plotted as Cp/T versus T. The vertical arrow shows the Néel temperature (TN) of BiCr0.6Ga0.4O3. The insert gives a fragment of the Cp/T versus T curves of BiCr0.5Ga0.5O3, BiCr0.4Ga0.6O3 and BiCr0.3Ga0.7O3.

Figure 9.

Real (χ′ versus T) and imaginary (χ″ versus T) parts of the ac susceptibility curves of BiCr1−GaO3 (R3c) with x = (a) 0.4, (b) 0.5, (c) 0.7 and (d) 0.6.

No characteristic λ-type anomaly is observed on specific heat of BiCr0.5Ga0.5O3. But we observe an excess of magnetic specific heat between about 20 and 80 K in BiCr0.5Ga0.5O3 in comparison with BiCr0.3Ga0.7O3 (figures 8 and S15 of the ESI), which comes from magnetic interactions between Cr3+ ions. A low temperature, the tail on the specific heat increases with increasing x in BiCr1−GaO3 probably from short-range magnetic interactions. Despite the absence of specific heat anomalies, magnetic properties of BiCr0.5Ga0.5O3 and BiCr0.4Ga0.6O3 are very similar with those of BiCr0.6Ga0.4O3, but with transitions at lower temperatures of 36 and 18 K, respectively. In particular, the χ′ versus T and χ″ versus T curves of BiCr1−GaO3 with x = 0.6, 0.5 and 0.4 are remarkably similar to each other (figure 9) demonstrating sharp and frequency-independent anomalies at TN. Therefore, we can assume that BiCr0.5Ga0.5O3 and BiCr0.4Ga0.6O3 also have long-range magnetic ordering at TN = 36 and 18 K, respectively. Because of the highly diluted magnetic sublattice, the entropy change associated with the transitions is very small resulting in the absence of specific heat anomalies. Note that long-range magnetic order was found in BiFe0.5Sc0.5O3 [22] and LaMn0.4Ga0.6O3 [23] by neutron diffraction, thus, confirming that long-range magnetic ordering can occur in samples with highly diluted magnetic sublattices.

Magnetic properties of BiCr0.3Ga0.7O3 are principally different (figures 7 and 9(c)). BiCr0.3Ga0.7O3 demonstrates basically paramagnetic behaviour as can be clearly seen from the χ′ versus T curve and the absence of any anomalies on the χ″ versus T curves (figure 9(c)). BiCr0.3Ga0.7O3 still has a large Curie–Weiss temperature of about −73 K (table 1) indicating strong short-range AFM coupling between Cr3+ ions, but there should be no long-range ordering because x = 0.7 is above the percolation threshold of x = 0.69 for perovskite structures [24]; below the 31% concentration, a dopant cannot be linked in a continuous path throughout a crystal. We note that BiCr0.3Ga0.7O3 and other samples with x = 0.4–0.6 show a small divergence between the ZFC and FC curves below about 90 K; this feature most probably originates from trace amounts of a magnetic impurity (figures S5 and S6 of the ESI).

Isothermal magnetization curves of BiCr1−GaO3 with x = 0.4, 0.5, 0.6 and 0.7 are given in figure 10. They show that a weak ferromagnetic moment is developed below TN in BiCr1−GaO3 with x = 0.4, 0.5 and 0.6. In BiCr0.6Ga0.4O3, the magnetization reaches 0.17μB/Cr at 5 K and 50 kOe; at 5 K, the remnant magnetization is 0.023μB/Cr, and the coercitive field is about 180 Oe (figure S11 of the ESI).

Figure 10.

Isothermal magnetization curves of (a) BiCr0.6Ga0.4O3 (R3c) at 5, 30 and 70 K, (b) BiCr0.5Ga0.5O3 (R3c) at 5, 30 and 70 K, (c) BiCr0.4Ga0.6O3 (R3c) at 5 and 30 K and (d) BiCr0.3Ga0.7O3 (R3c) at 5 and 30 K.

The resulting magnetic phase diagram of BiCr1−GaO3 is given in figure 11. The TN gradually decreases with increasing x, as one would expect because of the dilution of the magnetic sublattice by a nonmagnetic ion, while TSR increases. By the extrapolation, the TN and TSR should merge near x = 0.15. There is almost linear dependence of TN on x in the compositional ranges of 0.0 ≤ x ≤ 0.3 and 0.4 ≤ x ≤ 0.7. In both ranges, TN vanishes near x = 0.7 by the extrapolation, that is, near the percolation threshold. From the extrapolation, we can also estimate that TN should be about 130 K for a hypothetical R3c phase of BiCrO3, which was studied theoretically in some papers [16, 25]; the theoretically estimated TN is about 80–120 K [25]. The G-type AFM structure should realize in BiCrO3-based perovskites as predicted in many theoretical papers [15, 16, 25] and found experimentally [11-13]. However, spin canting mechanisms might be different depending on the symmetry; this is why different regions are marked as c-AFM1, c-AFM2 and c-AFM3 in figure 11. Spin canting is allowed by the symmetry in the C2/c and R3c structures and G-type magnetic arrangements.

Figure 11.

Magnetic phase diagram of BiCr1−GaO3. TN is the Néel temperature, TSR is temperature of a spin-reorientation transition, PM is a paramagnetic phase and c-AFM is a canted antiferromagnetic phase.

In conclusion, we investigated magnetic properties of BiCr1−GaO3 solid solutions. AFM order with weak ferromagnetism is observed below TN = 54, 36 and 18 K in the samples with x = 0.4, 0.5 and 0.6, respectively, having a polar R3c structure. TN decreases from 111 K in BiCrO3 to 98 K in BiCr0.9Ga0.1O3, and the spin-reorientation transition temperature increases from 72 K in BiCrO3 to 83 K in BiCr0.9Ga0.1O3, having C2/c symmetry. A magnetic phase diagram of BiCr1−GaO3 is constructed.